Serial communication networks provide many advantages over other well-known networks such as multipoint, star or mesh networks, the most valuable being a fair distributed polling function which readily supports peer-to-peer communications amongst a large number of stations with a high utilization of the network capacity.
One major drawback of the serial network is its propensity to catastrophic failure when any component of the network fails. The drawbacks of a serial network are somewhat reduced by use of a dual ring communication system. A dual ring communication system is comprised of a primary ring or data path and a secondary ring or data path. In normal operation information is passed in a first direction along the primary ring. No information is passed along the secondary ring until a failure occurs. When a failure occurs the secondary ring may be connected to the primary ring to complete the serial communication network. In such a case information is transmitted along the secondary ring in a second direction opposite that of the primary ring.
Over the years many techniques have been developed to detect and/or isolate faults in serial communication networks. One of the more useful techniques, which is in use today in the IEEE 802.5 Token Ring, is disclosed in U.S. Pat. No. 3,564,145. This technique known as beaconing identifies a station, by its address, immediately downstream of a failed network component or station. In a static network (e.g. one in which the network topology is fixed or known) corrective action can be taken to bypass or fix the failed network component.
Another technique (Dual Ring Reconfiguration) has proven very useful in the isolation of faults in a serial network, thereby providing complete or partial network operation following the failure of a network component. This technique employs the dual serial rings described above which may be converted to single ring via switching means to thereby bypass a failed network component. The patents listed below disclose a variety of Dual Ring Reconfiguration implementations:
U.S. Pat. No. 3,519,750 PA1 U.S. Pat. No. 3,876,983 PA1 U.S. Pat. No. 4,009,469 PA1 U.S. Pat. No. 4,354,267 PA1 U.S. Pat. No. 4,390,984 PA1 U.S. Pat. No. 4,527,270 PA1 U.S. Pat. No. 5,538,264 PA1 U.S. Pat. No. 4,594,790 PA1 U.S. Pat. No. 4,709,365 PA1 U.S. Pat. No. 3,458,661 PA1 U.S. Pat. No. 4,035,770 PA1 U.S. Pat. No. 4,048,446 PA1 U.S. Pat. No. 4,245,343 PA1 U.S. Pat. No. 4,763,329
The patents listed below disclose a variety of manual and automatic techniques for bypassing a failed network component in a single ring serial network:
While all of the techniques described above are useful either by themselves or in combination, they may have difficulty in providing fast, efficient or complete restoration of communications in a serial ring network following failures of all kinds.
Modern serial networks such as the IEEE 802.5 Token Ring Network generally employ many (up to several hundred or more) ports located throughout an establishment. Many of these ports are not utilized or are connected to inactive stations. In addition, stations (each of which includes a unique identity or address) are frequently moved from one port to another for the convenience of the operator.
In view of the mobility of the stations and the large number of ports which have no or inactive stations connected, the station identity or address accompanying a beacon message provides little information to locate the geographic position of the failed network component.
The technique (Next Active Upstream Neighbor) disclosed in U.S. Pat. No. 4,507,777 is very useful in managing fault recovery in serial networks; however, the sequential station identities or addresses derived from this technique do not provide sufficient network topology information to accurately pinpoint the physical location of the failed component. For example, two adjacent active stations may be separated on the physical network by a number of non-connected or inactive ports. Thus, knowing that station X detected a failure and that station C preceded X does not physically locate a particular faulty component.
U.S. Pat. No. 5,132,962 describes a reconfiguration unit capable of fault isolation and bypass in a dual ring communication system. While this unit fully implements the IEEE 802.5 fault isolation and bypass procedures, it requires three adapters to interface to the primary and secondary rings of the dual ring communication system. These adapters are typically the single most expensive component of the reconfiguration unit and, therefore, a unit requiring three such adapters may be impractical in certain applications where the high degree of error recovery cannot justify the cost of such reconfiguration units. These reconfiguration units utilize multiple adapters and have no need of determining the nearest downstream units address because the multiple adapter implementation does not require such information to carry out fault isolation and bypass.
Unfortunately, the methods and apparatus described above do not provide a method for determining the address of the nearest downstream reconfiguration unit in a serial network. Furthermore, because many existing dual ring networks have been implemented with the technology described above any methods of determining the address of the nearest downstream reconfiguration units should be compatible with such networks. Furthermore, any systems capable of determining the nearest downstream reconfiguration unit should be compatible with existing dual ring communication systems. To assure compatibility with future designs of reconfiguration units and networks it would be desirable that any method or system of determining the address of the nearest downstream reconfiguration unit be compatible with a network protocol such as the IEEE 802.5 Token Ring standard.